The present invention relates to an electroluminescent device which is capable of directly converting electric energy of an electric field applied thereto into light energy and capable of producing a large surface-area luminescence different from conventional incandescent lamps, fluorescent lamps and light-emitting diodes.
Electroluminescent devices produced by laminating thin films of inorganic compounds are known. In such inorganic thin-film electroluminescent devices, a transparent electrode (ITO), an insulating layer (Si.sub.3 N.sub.4), a luminescent layer (ZnS : Mn), an insulating layer (Si.sub.3 N.sub.4) and a metal electrode (Al) are laminated in sequence on a glass substrate. Although such inorganic thin-film electroluminescent devices have high luminance, they need a high driving voltage of from 100 to 200 V, and therefore, the use of an exclusive IC which withstands a high voltage force is required. Furthermore, a matrix material for the luminescent layer and a material used as an active agent are limited, and it is not always possible to obtain an electroluminescent device having high luminance at a desired wavelength.
In recent years, attempts have been made to manufacture an electroluminescent device in which organic thin films are laminated.
Such electroluminescent devices are disclosed in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 194393/1984. This electroluminescent device is composed of an anode, a hole transporting zone, an organic luminescent zone, and a cathode, in which the organic electroluminescent zone is less than 1 .mu.m, and either of the anode or the cathode is capable of permeating at least 80% of radiation rays at a wavelength of 400 nm or above, and either has a power conversion efficiency of 9.times.10.sup.-5 W/W. As hole transmitting compounds constituting the hole transmitting layer, 1,1-bis(4-di-p-tolylaminophenyl)-4-phenyl-cyclohexane, 1,1-bis(4-di-p-tolylaminophenyl) cyclohexane, 4,4"-bis (diphenylamino) quatriphenyl, bis(4-dimethylamino-2-methylphenyl) phenylmethane, and N,N-tri(p-tolyl) amine may be exemplified.
Japanese Patent Application Laid-Open (KOKAI) No. 51781/1982 discloses an organic electroluminescent device comprising an anode, a cathode, an electroluminescent zone sandwiched therebetween which contains at least one type of organic electroluminescent substance and a binding agent having a breakdown voltage of more than about 10.sup.5 V/cm, and a hole transporting zone sandwiched between the electroluminescent zone and the anode, including a porphyrin compound layer. As the porphyrin compounds of the hole transmitting compounds, phthalocyanine and metallic phthalocyanine of cobalt, magnesium, zinc, palladium, nickel, copper, lead or platinum may be exemplified.
In the above-described organic thin-film electroluminescent devices, as compared with the inorganic electroluminescent, the choice in materials for the luminescent layer has been carried out on a large scale and materials capable of light emission of various wavelengths have been found. Furthermore, since the organic thin-film electroluminescent devices have generally a driving voltage force from 5 to 60 V and facilitate large surface area luminescence. So, application of the electroluminescent device to various types of electroluminescent or display devices including a full-color display, has been expected.
However, researches on the electroluminescent devices using organic compounds as a luminescent material have not been gone deep into and it can be said that sufficient studies with respect to the materials and device-forming techniques have been made. So, there are many problems with respect to improvement in luminance, control of the wavelength of the light emission, and improvement in durability.
As a result of the present inventors' earnest studies with respect to at least two organic compound layers sandwiched between two electrodes for providing an electroluminescent devices which can be driven at a low voltage, maintain its luminosity for a long period, control easily on the wavelength of the light emission and has excellent durability, it has been found that an electroluminescent device having at least one of the organic compound layers comprising as a hole transporting substance an organic compound represented by the following general formula, can drive at a low voltage and can provide an emission light of high luminance for a long period. ##STR3## (wherein R.sup.1 represents hydrogen atom, a lower alkyl group, a lower alkoxy group, a halogen atom or nitro group, and A represents ##STR4## in which R.sup.2 represents a lower alkyl group, a substituted or non-substituted aryl group or an aralkyl group, and R.sup.3, represents a lower alkyl group.) The present invention has been attained based on this finding.